1. Field of the Invention
The invention relates to a wire saw and a process for cutting shaped articles from a workpiece.
2. The Prior Art
Wire saws are used to cut a multiplicity of semiconductor wafers from a crystal in one operation. Wire saws of this kind have a wire web, which is formed by a sawing wire wound round two or more wire-guide rollers. The sawing wire may be covered with a coating. With wire saws having a sawing wire which does not have fixedly bonded abrasive grain, abrasive grain is supplied during the cutting operation in the form of a suspension or slurry. During the cutting operation, the workpiece passes through the wire web, in which the sawing wire is arranged in the form of parallel adjacent wire segments. The workpiece is made to pass through the wire web by a feed device which guides the workpiece onto the wire web or the wire web onto the workpiece. It is advantageous for the direction of movement of the wire segments to be changed constantly by periodically changing the direction of rotation of the wire-guide rollers, i.e., the oscillating operation, in order to allow the cutting slurry to be supplied from both sides. In this way, it is possible to produce wafers of improved geometry (TTV, warp).
The thickness of the cut shaped articles is fixed by selecting the distance between the parallel wire segments. This distance is limited as one moves toward smaller amounts by the fact that the sawing wire runs in grooves formed in the wire-guide rollers. The grooves require comparatively large amounts of space and cannot be arranged as close together as desired without impairing their wire-guiding properties. The thickness of semiconductor wafers should therefore not fall below a certain minimum thickness, even though this might be useful to conserve material. Furthermore, the wire saws described can generally only be used to produce disk-like shaped articles having parallel side faces.